According to the conventional VAD method (vapor-phase axial deposition method), known as a means for manufacturing a quartz optical fiber preform, glass forming(glass synthesizing) is carried out by a vapor phase reaction of a metal halide in an oxyhydrogen flame, and the thus-generated glass fine particles are deposited on the periphery of a target member, to obtain an optical fiber soot. The target member is pulled up in an axial direction, while the distance between a position of the tip of a soot and a core burner is made constant. The rate for pulling the soot up is referred to as the growth rate. After that, a glass porous soot, which is manufactured in this manner, is sintered, to give a transparent preform, by a high-temperature heat treatment, and then the preform is processed into an optical fiber by drawing or the like.
In manufacturing an optical fiber soot by these methods, the target member and the burner are contained in a reactor, so that the glass fine particles, generated in the flame of the burner, are adhered to the surface of the target member evenly and efficiently, and that the flow of air is regulated by the covering reactor.
An apparatus for manufacturing an optical fiber soot, which has been used in this method, is configured as shown in FIG. 7(a) and FIG. 7(b). FIG. 7(a) is a schematic front view of the apparatus for manufacturing the optical fiber soot, and FIG. 7(b) is a side view of FIG. 7(a), viewed from the direction of an arrowhead A. As shown in FIG. 7(a) and FIG. 7(b), a reactor 12 is provided with multiple tubes of a core burner 1 and a clad burner 2. Then, by injecting a silicon tetrachloride gas in oxyhydrogen flames (core flame) 3, which flame is generated from the core burner, and silicon dioxide fine particles are formed by a flame hydrolytic reaction. The resultant particles are deposited, in the longitudinal direction, onto a pilot bar 4, to obtain a porous soot 5. In this case, if a small amount of addictive, such as germanium tetrachloride, phosphoryl chloride, or boron bromide is injected together with silicon tetrachloride gas in the flame, fine particles composed, for example, of germanium dioxide are synchronously generated with silicon dioxide fine-particles, so that it is possible to manufacture a porous soot having a predetermined distribution of germanium dioxide or the like in the radius direction of the porous soot.
According to a manufacturing process of the optical fiber soot by using the conventional VAD method, as shown in FIGS. 7(a) and 7(b), in order to evenly and efficiently deposit the glass fine particles, and to prevent the reactor from being over-heated and the glass from being deposited to the wall of the reactor, the glass fine particles are deposited onto the pilot bar, by rotating and pulling up the optical fiber soot in a regulated airflow, such as horizontal gas flow 6 and an air curtain flow 7, which flow from air inlets 14 and 15 on the burner side, to an air-discharging pipe 11, and a descending gas flow 8, or the like. The air inlet 14 represents an air inlet for the horizontal gas flow, and the air inlet 15 represents an air inlet for the air curtain flow. Additionally, as shown in FIGS. 7(a) and 7(b), the temperature of the soot is controlled by monitoring the surface temperature of the soot, using a thermos viewer 9, during synthesis of the glass. Further, in the same way, the temperature of the tip of the soot, which is important for growth of the soot, is also controlled by monitoring with a radiation thermometer 10.
However, in order to manufacture more homogeneous optical fiber soot and to improve productivity of optical fiber soot, in forming the soot during the horizontal gas flow and the descending gas flow, it was found that the following problems were involved.                a) As is shown by arrows in FIGS. 8(a) and 8(b), ascending gas flow, generated from the core flame and the clad flame, is pushed down by the descending gas flow. Thus a turbulent descending airflow is caused. The descending airflow falls on the bottom surface and the side surface of the reactor, and then generates an ascending airflow. The degree of the flicker of the core flame is increased by the ascending airflow, so that the rate of glass growth is made instable. As a result, the thus-obtained optical fiber soot lacks uniformity in a diameter of the longitudinal direction.        b) Since the degree of the flicker of the core flame is so large, the temperature of the tip of the soot becomes inconstant. Therefore, the soot density at the tip portion of the soot is lowered, and cracks occur easily on the soot.        